The invention relates to thyristors and more particularly to sensitive thyristors, that is to say capable of being caused to conduct by a low gate current (typically less than 500 mA).
These thyristors have the advantage of requiring a low control power but this advantage may become a disadvantage to the extent that these thyristors risk being triggered prematurely by parasite currents even small ones.
In particular, sensitive thyristors very often have poor resistance to parasite currents generated in the semiconductor body in the presence of high voltage time gradients (high dV/dt) between anode and cathode: they also have poor temperature resistance, the generation of electronhole pairs due to the heat creating parasite currents capable of causing the thyristor to conduct.
To try to improve the resistance to premature triggering due to high dV/dt or due to the temperature, up to now solutions have been adopted which consist in adding between the gate and cathode a decoupling resistance external to the thyristor, or sometimes an external decoupling capacity or a combination of a resistance and a capacity.
The purpose of the resistance is to allow the gate region to recover capacitive currents due to the dV/dt or the thermal carrier generation currents without imposing a priori the injection of the gate-cathode junction but it reduces the sensitivity of the thyristor. However, even when the gate is connected, this resistance does not prevent a current path from being formed between the gate and the cathode which will be associated as a function of the dV/dt or of the temperature with an internal lateral current flow between the part situated under the cathode and the gate this lateral flow internal to the thyristor may cause triggering of the thyristor. This phenomenon is all the more critical the wider the cathode (powerful thyristors). It may be partially avoided by reducing the elementary cathode widths through interdigitation of the gate and the cathode but that assumes that there is interdigitation absolutely everywhere which does not allow widened cathode zones to be provided for making contact. The technology of mounting the electrodes is therefore made difficult.
The use of a capacity external to the thyristor, between gate and cathode, allows the parasite current generated by the dV/dt voltage gradients to be absorbed but it has no effect on the thermally generated carriers and in addition it introduces a tripping delay during voluntary enabling of the thyristor.
The present invention proposes a new sensitive thyristor structure which considerably improves the immunity to parasite triggering by anode -cathode voltage gradients (dV/dt) or by a temperature rise, and without using the conventional concept of interdigitation of the cathode.